The present invention generally relates to machining equipment and processes, and more particularly to an apparatus and process for machining titanium and its alloys.
Titanium and its alloys have properties that are quite distinct from other high temperature metal alloys, including nickel-, cobalt-, and iron-based (e.g., iron-nickel) superalloys. For example, titanium alloys (alloys that contain titanium as their predominant constituent) exhibit high strength to weight ratios, good temperature and chemical resistance, and relatively low densities. which make them ideal for applications in the aerospace industry. A particularly notable example is Ti-6Al-4V, whose nominal composition is, by weight, about 6% aluminum, about 4% vanadium, the balance titanium and incidental impurities. The Ti-6Al-4V alloy is extensively used in the aerospace industry due to its excellent properties of high specific strength and corrosion resistance. However, titanium alloys are extremely difficult to machine, and costs associated with their machining are high due to short tool life.
Several explanations for the poor machinability of titanium alloys have been proposed. Titanium has low thermal conductivity, which impedes heat transfer out of the cutting zone while creating high cutting zone temperatures. Titanium also exhibits a high chemical affinity toward cobalt binders found in most cutting tool materials. The interface between the cutting tool and chips produced by a machining operation is usually quite small, which results in high cutting zone stresses. There is also a strong tendency for titanium chips to pressure-weld to cutting tools. Due to these issues, conventional wisdom has been that only uncoated carbide tools, such as WC/Co cermets, are suitable for machining titanium alloys. Because tool-chip interface temperature increases approximately proportional to the square root of cutting velocity and carbide tools quickly lose strength as the temperature goes above several hundred degrees Celsius, tool temperature has limited cutting speeds of titanium using carbide tools to typically less than 60 meters per minute (m/min).
It is well known that heating a workpiece can weaken the workpiece and consequently make machining easier. Laser-assisted machining (LAM) and plasma-enhanced machining (PEM) have both been used to improve the machinability of various materials. However, LAM and particularly PEM also elevate the cutting tool temperature. When LAM and PEM has been used with uncoated carbide tools, improvements in tool life are minimal due to the elevated tool temperature during machining. Therefore, more advanced tool materials, such as ceramics and cubic boron nitride (CBN), are typically employed with LAM and PEM to enable the cutting tool to survive under high temperatures. However, titanium alloys particularly exhibit chemical affinity toward more advanced materials such as ceramic and CBN tools used with LAM and PEM. There have been previous studies on employing LAM techniques when machining titanium, though tool life improvements have not been reported with LAM.
Tool wear during machining of titanium using a carbide tool is governed by diffusion wear, which depends in part on the interface temperature between the cutting tool and the chip removed by the tool. Conventional wisdom has been that reduced tool wear resulting from diffusion wear can be achieved by cooling both the tool and the workpiece to reduce the interface temperature. For this purpose, conventional cooling fluids and more advanced cooling techniques, such as cryogenic-enhanced machining (CEM), have been used. CEM makes use of a cryogenic fluid, such as liquid nitrogen, which is typically directed over the cutting tool so that the fluid also overflows the workpiece. Consequently, conventional wisdom has been that the combination of CEM and either LAM or PEM is counter intuitive because they appear to offset the purpose and advantages of each other.
Due to advantages associated with its mechanical and physical properties, there is a desire for processes and equipment capable of achieving higher removal rates for titanium alloys.